Mozilla today delivered a re-engineered Firefox, branded "Quantum," that it claims is twice as fast as just a few months earlier, and unveiled a re-drawn UI (user interface) with minimalist leanings.

The browser maker also dumped Yahoo as Firefox's default search engine in the U.S., Canada and elsewhere, to return to Google, its partner - and primary financier - before a falling out three years ago.

"It's by far the biggest update we've had since we launched Firefox 1.0 in 2004," Mark Mayo, the Mozilla executive who oversees the browser, wrote in a Tuesday post to a company blog.

The new browser's official designation is Firefox 57, just another in an every-six-week series of upgrades. Until now, Mozilla has always numbered, never named, its browser releases.

Firefox Quantum boasts a revamped rendering engine with a new CSS (cascading style sheets) layout engine. Along with other components, the engine was written in Rust, a programming language that originated in Mozilla's research group. The result: a significant speed increase.

"Firefox Quantum is over twice as fast as Firefox from 6 months ago, built on a completely overhauled core engine with brand new technology stolen from our advanced research group," said Mayo, who credited the rendering engine's use of multiple processor cores for the boost.

Other changes ran down the page and onto the next in Firefox's release notes, and included active tab prioritization and a switch-over from legacy add-ons to those built using the WebAssembly API (application programming interface).

Quantum also sports a new UI, the first major redesign since 2011's Firefox 4. The UI and user experience (UX) changes, derived from an ongoing project tapped as "Photon," emphasize speed improvements, both real and simply perceived.

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Firefox 57, aka "Quantum," sports a minimalist UI -- finally, a merge of the address and search bars, making it the last browser to make that move -- and under the skin, significant performance improvements.

The UI matches the streamlined austerity of rivals such as Google's Chrome and Microsoft's Edge. Firefox 57 combines the address and search bars - for new installations at least; existing users who miss the latter can restore it - reduces the clutter at the top of the window, and debuts a reworked new tab page.

Almost lost among the visual and technological changes was a simultaneous announcement that after three years, Firefox is switching back to Google as the default search engine in the U.S., Canada, Taiwan and Hong Kong. Other markets remain with their previous defaults, including Yandex (in Russia and Belarus) and Baidu (People's Republic of China).

"This is part of our ongoing search strategy, announced in 2014 to evaluate and select the best search experience in each region as opposed to having a single global default," asserted Denelle Dixon, Mozilla's chief legal officer, in a separate blog post.

What Dixon left unsaid was reported last year, when Recode and the New York Times claimed that the 2014 Yahoo-Mozilla contact gave Mozilla the right to switch to another search provider if Yahoo was sold. It was, to Verizon, which this summer completed a $4.5 billion acquisition of the former Internet giant. According to those news reports, Mozilla would still be due money, perhaps more than $1 billion although the amount was unclear, for the remaining two years of the contract.

Mozilla's return to Google as the default in the U.S. and Canada - the same markets Yahoo had sewed up with its 2014 pact - implied that it had walked away from the latter search engine.

The Quantum project, the Photon UI overhaul and last year's move to make Firefox a multiple-process browser have been crucial to Mozilla's recovery strategy. Firefox once accounted for a quarter of the world's user share, but dipped as low as 8% in the summer of 2016. Since then, it's scratched back to 13% as of October.

"No. I just can't see it at this point," said Jack Gold, principal analyst at J. Gold Associates. "The dominance of Google on Android, of Safari on iOS, Internet Explorer and Chrome on PCs, there's just not room for another browser. It would have to be so unique."

And Firefox's Quantum upgrade isn't that, Gold argued. "I just don't see how they can come back. It's a futile effort."

With virtually no relevance in mobile, even with Android and iOS versions of Firefox available, and failed campaigns to build its own mobile OS or transform that into an Internet-of-things (IoT) platform, Mozilla has little strategic wiggle room.

"In a year from now, it will be the same all over again," Gold said, referring to Firefox's minor player status.